The goals of this application are to study the heterosexual transmission of HIV in the Greater Providence area in order to: 1) estimate HIV seroprevalence and the incidence of seroconversion among heterosexual men and women whose only known risk factor is sexual contact with someone who is HIV seropositive or is at increased risk for becoming infected with HIV; 2) identify sexual practices, including contraceptive use that facilitate or impede HIV transmission; 3) identify determinants (e.g. other STD's that influence the risk of infectivity; 4) describe the natural history of HIV infection among seropositive heterosexual men and women; 5) compare sexual practices and immunologic status of couples that are concordant (same HIV serologic status) versus those that are discordant; 6) utilize Western Blot and radioimmune precipitation assays serially to follow specific patterns of HIV antibody responses and correlate these with clinical status; 7) utilize viral cultures to elucidate the relations of viremia and the presence of HIV in genital secretions with the ability to infect partners and clinical progression of HIV infection; 8) study the differential serologic response between concordant and discordant heterosexual partners to determine if neutralizing antibodies against free virus or antibody dependent complement mediated cytotoxicity against infected cells plays a role in viral transmission; 9) develop a repository of sera and white cells that can be used to validate future diagnostic and prognostic assays; 10) develop a data base regarding heterosexual sexual practices in this region that can be analyzed to develop future interventions to decrease the spread of HIV. Subjects will be recruited from college campuses and from programs that assist high risk heterosexuals, such as methadone maintenance centers; sexually transmitted disease clinics and HIV antibody screening centers; physician referrals; AIDS-oriented community-based groups. Four thousand five hundred persons will be screened for HIV antibodies over 2-3 years and about 1/4 will be enrolled utilizing an algorithm to detect individuals at high risk for HIV infection who will participate in the more extensive ongoing study which will include a detailed behavioral questionnaire, a physical examination, and blood collection. Participants selected for the ongoing study will be seen every four months in the first year and then twice a year for the next four years.